In Carlson-type xerography, a photoreceptor comprising a layer of photoconductive insulating material deposited over a conductive substrate is used to support electrostatic latent images. The surface of the photoconductive material is electrostatically charged and exposed to a light pattern of an image to be reproduced, to selectively discharge the charged surface in accordance with the image. The undischarged areas of the surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. The process is well known, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals.
The photoconductive material in photoreceptors commonly used in xerographic devices often comprises selenium or its alloys. Charging of selenium-based photoreceptors commonly requires a positive charging corona device such as a corotron or scorotron driven with a positive D.C. voltage signal. In practice, the positive charging corona device comprises a strand of bare tungsten or oxidized tungsten wire forming a coronode supported between two insulating blocks, and driven at high D.C. voltage in the range of 3500-10,000 volts. The corona charging device is arranged closely adjacent to the surface of the photoreceptor and is designed so as to apply a uniform charge thereto. during relative movement of the photoreceptor and the corona charging device.
During operation of the positive charging corona device to charge the photoreceptor to a uniform voltage level, a copy quality defect known as "pepper tracking" is sometimes observed. Pepper tracking, as the name implies, is a copy quality defect which results in a large number of undesirable small spots in a narrow area forming a band across the copy sheet. The band or track generally occurs along a narrow path parallel to the direction of sheet travel. When pepper tracking is observed, it is sometimes seen with several such bands. The spots on the copy may be in the form of dark spots on a white background or white spots on a dark dusting. Both arise as a result of excessive deposition of positive ions in small localized regions on the photoreceptor. The source of this excess ion deposition is so called "hot spots" on the coronode. These hot spots represent local microscopic regions of the coronode which, for reasons which are poorly understood, emit bursts of positive ions in pulses or streamers which are on the order of microseconds in duration. They may be compared to an electrical arc which is self-limiting.
It is believed that the hot spots and the associated pepper tracking defects are primarily the result of operating the corona charging device in an environment in which certain kinds of contamination are present. Particularly, certain environmental conditions cause a defect in the bare wire coronodes commonly used in positive charging corona devices. The defect has been particularly observed in areas having a high contamination level of silicon-based contaminants, such as silicon oil vapor. Silicon-based contaminants are frequently encountered in the environment, and silicon oil vapor is pervasive in many xerographic machines in which silicon oil is used as a release agent on the fuser rolls. Under these conditions, a glassy-like substance, believed to be a silicon oxide, is observed to build up on the coronode in areas where the streamers are believed to originate. The build up often occurs in the form of defects on the coronode surface. Morphologically, these defects have the appearance of cones, whiskers and rosettes. Electrical arcing, or streamers, have been observed emanating from the areas adjacent to these defects. However, rather than burning a hole in the photoreceptor, the streamers produced by this condition appear to be self-limiting in life and end, usually before doing damage to the photoreceptor. The result is that a small area of excessive positive charge is present on the photoreceptor. In those areas of the photoreceptor not discharged during the exposure step, the excess charge levels are frequently high enough to cause breakdown to the magnetic brush development roll. This results in white spots on an otherwise dark area on the developed image. In those areas of the photoreceptor discharged to background potential, regions in which development of toner normally does not occur during the development step, the excess charge areas are insufficiently discharged to prevent development of a toned image and therefore dark spots are observed in these regions against an otherwise white background.
A typical service response to the pepper tracking defect is to replace the coronode wire in the corona charging device. This is time consuming, and necessarily expensive. In addition, there exist environments where the level of airborne contamination is sufficiently high that wire replacement serves to eliminate the pepper tracking problem for only a relatively short time. The use of platinum wire as a substitute for tungsten or oxidized tungsten may have minor beneficial effects in retarding the occurrence of pepper tracking, but its effectiveness is limited, and as a precious metal, it is expensive.
The pepper tracking defect is also observed as a problem in other positive charging applications where a corona charging device is used to apply a positive charge to a charge retentive surface, such as the electroreceptor in ionography. While tungsten and platinum are materials of choice for coronode wire in most positive charging applications, other materials may be used, and pepper tracking is observed with such materials to various degrees.
A.C. driven corona charging devices are known for various charging arrangements. Thus, for example, in charging processes requiring multiple charging functions, for charging a photoreceptor having an insulating overcoating, such as that shown in U.S. Pat. Nos. 4,565,436 to Okada et al and 4,339,783 to Kinashi et al, a secondary corona device is driven with an A.C. signal. Variations of the positive or negative portions of the A.C. signals driving these devices are shown for the purpose of obtaining greater uniformity in the multiple step charging process. Scorotron charging devices may also be driven with an A.C. signal to the coronode and a D.C. bias applied to the screen, as shown in U.S. Pat. Nos. 2,777,957 to Walkup; 2,879,395 to Walkup; 3,370,212, to Frank; and 3,390,266 to Epping. Corona charging devices driven with an A.C. signal, and having a D.C. bias level applied to the signal are taught in U.S. Pat. Nos. 3,076,092 to Mott; 4,456,365 to Yuasa; and Weber 4,306,271. The A.C. voltage signal may be rectified to provide a shaped wave to the corona wire, as shown in U.S. Pat. No. 3,800,154 to Tanaka.